JP60-37455A discloses a continuously variable transmission in which a two-forward speed subtransmission mechanism is provided in series with a belt continuously variable speed change mechanism (to be referred to hereafter as a “variator”), and a gear position of the subtransmission mechanism is changed in accordance with an operating condition of a vehicle. Thus, an achievable speed ratio range is enlarged without increasing the size of the variator.
JP5-79554A discloses a technique for suppressing variation in an overall speed ratio (to be referred to hereafter as a “through speed ratio”) of this type of continuously variable transmission including a subtransmission mechanism by rapidly downshifting the variator when changing the gear position of the subtransmission mechanism from a low-speed gear position to a high-speed gear position. In so doing, JP5-79554A suppresses speed variation in an engine and a torque converter during a shift in the subtransmission mechanism, thereby preventing shift shock caused by inertia torque from the engine and torque converter.
Further, as disclosed in JP2002-106700A, during shift control in a continuously variable transmission, a speed ratio to be realized under current operating conditions is set as a destination speed ratio, a target speed ratio, which is an intermediate target value for causing an actual speed ratio to follow the destination speed ratio at a predetermined transient response, is set, and the continuously variable transmission is controlled such that the actual speed ratio becomes the target speed ratio.